docs: def all_keys(self, uppercase_keys=False):
          """Return all keys regardless where they are set."""
          d = {}
  
          for k in self._override.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          for k in self._args.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          for k in self._env.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          for k in self._config.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          for k in self._kvstore.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          for k in self._defaults.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          for k in self._aliases.keys():
              d[k.upper() if uppercase_keys else k.lower()] = {}
  
          return d.keys()